An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor may be classified as a charge coupled device (CCD) and a complementary metal oxide silicon (CMOS) image sensor (CIS).
Such an image sensor may be include a photodiode for sensing irradiated light and a logic circuit unit for processing the sensed light into an electrical signal and concert it into data. The greater capacity the photodiode has for receiving light, the better the photosensitivity characteristics of the image sensor.
In order to enhance such photosensitivity, a technique may be for enlarging a fill factor of the area the photodiode occupies among the entire area of the image sensor or condenses light into the photodiode by changing the optical path incident on the region other than the photodiode.
Such a condensing technique may include forming a microlens. A convex microlens may be formed on and/or over the uppermost surface of the photodiode using material having good light transmittance to refract the path of incident light so that light in greater quantities may be irradiated to the photodiode region. The light horizontal to the optical axis of the microlens may be refracted using the microlens so that the focus thereof is formed at a predetermined position on the optical axis.
An image sensor may include photodiode, an interlayer dielectric layer, a color filter layer, and a micro lens. The photodiode may perform the function of sensing and converting light into an electrical signal. The interlayer dielectric layer may perform the function insulating each metal wiring. The color filter layer can represent three primary colors of light such as red (R), green (G), and blue (B). The microlens may perform the function of condensing light into the photodiode.
As illustrated in example FIG. 1, an image sensor can include insulating layer 20 formed on and/or over semiconductor substrate 10 formed with a plurality of photodiodes 40. Color filter layers 30 representing red (R), green (G), and blue (B) corresponding to the plurality of photodiodes 40 may be formed on and/or over insulating layer 20. Planarization layer 25 for planarizing the uneven surface layers of color filter layers 30 may be formed on and/or over color filter layers 30.
A plurality if microlenses 50 each corresponding to the plurality of photodiodes 40 and color filter layers 30 may then be formed on and/or over planarization layer 25. Microlenses 50 may be formed in a convex lens pattern for collecting light to photodiodes 40 by patterning microlenses 50 using a photoetching process.
As illustrated in example FIG. 2A, photoresist 60, which is a material for microlens 50, may be coated on and/or over planarization layer 25.
As illustrated in example FIG. 2B, photoresists 60 may then be covered with mask 61 and then subjected to an exposure process using a defocus phenomenon so that photoresist 60 is patterned in a trapezoidal pattern.
As illustrated in example FIG. 2C, photoresists 60 in a trapezoidal pattern may then be heated up to a melting point and then reflowed. Subjecting to the reflow process, the photoresist pattern has mobility and is rounded so that microlens 50 is completed. Forming microlens 50 in this manner, however, may generate gap (G) between neighboring microlenses 50. In the microlens forming process, the gap between neighboring microlenses may have the largest effect on the performance of the image sensor.
The smaller the gap, the more the light sensitivity of the device is improved, sometimes as much as by 10 to 15% or more. Moreover, as the gap between the microlenses is smaller, a flux amount of light is large and the optical efficiency of light transferred to the lower end of a light diode in the device ma be increased.
In the case of a CMOS type device where a metal wiring may be provided in a pixel region, the light passes through the metal wiring arranged to avoid a path of light from the upper layer to the lower end to decrease the probability of it being scattered.
As described above, a microlens may be formed by a process patterning organic material in a form of a photoresist capable of being reflowed by using thermal energy at the place where the microlens is positioned on a planarization layer or a plane formed of the same material such as an oxide thin film, etc., using a lithographic method and then applying heat to reflow it.
When forming a microlens using such a process, since the width of the gap of the microlens may be determined using the gap of a pattern formed through a photolithographic before reflow, the minimum line width of the gap may be limited to 50 nm due to the limitation of lithographic resolution.
When making the gap of the microlens narrow below 50 nm by making the reflow excessive, since the flow of the microlens is determined using an equilibrium condition between surface tension and the reflow, it may be very difficult to control the generation probability of the lens bridge.
As illustrated in example FIG. 3, formation of a lens bridge may result in a mutual connection of neighboring microlenses. Consequently, it may be impossible to greatly reduce the size or otherwise eliminate the gap between neighboring microlenses.